Check TDR Rise Time Before Interpreting Closely Spaced PCB Features

A TDR trace can show that a discontinuity exists without resolving its exact geometry. Closely spaced features may merge into one response, and the measurement path can slow the incident edge. Interpret the trace using the system response that reaches the board, not only the instrument’s headline rise time.

Define the question being asked

Tektronix’s TDR primer distinguishes resolving adjacent discontinuities from detecting a single small feature and explains the role of rise time. Those are different measurement tasks. A quoted spatial-resolution number should therefore include the task and assumptions behind it.

State whether the objective is locating an open connection, comparing a via transition, measuring a uniform trace section or separating two nearby features. Identify the expected physical locations and approximate propagation path before assigning every trace bump to a component.

Include the connection path

Record cables, probes, launch structures, calibration and any filtering applied to the waveform. Determine the effective edge at the relevant reference plane using an appropriate verification method. Long or lossy connections can reduce the ability to distinguish features deeper into the path.

Keep round-trip timing separate from one-way distance when translating a reflection into a location. Use a propagation model appropriate to the actual structure. A generic dielectric value can provide an estimate, but it should not be reported as an exact fault coordinate.

Use controlled comparisons

Compare the suspect board with a known reference of the same construction and connection arrangement. Align the traces using a defined reference feature. Record differences in connector seating or fixture setup before interpreting a small local change as a manufacturing defect.

If filtering is used to emulate a slower application edge, retain the original and filtered results with their settings. The slower view can help assess relevance to the operating signal, while the faster view can expose details useful for diagnosis. They answer related but different questions.

Bound the physical conclusion

A broad response near two adjacent vias may justify further inspection, but it may not identify which via changed. Use complementary evidence such as layout coordinates, other port measurements or physical analysis when the TDR resolution is insufficient. Avoid assigning a precise impedance to a feature that the system cannot resolve cleanly.

Retain the measured rise-time context, reference plane, distance assumptions and comparison traces with the report. State whether the evidence detects, localizes or quantitatively characterizes the feature. This prevents a useful diagnostic waveform from being stretched into a more precise geometric or impedance claim than the setup supports.